mm/uffd: move USERFAULTFD configs into mm/

We used to have USERFAULTFD configs stored in init/.  It makes sense as a
start because that's the default place for storing syscall related
configs.

However userfaultfd evolved a bit in the past few years and some more
config options were added.  They're no longer related to syscalls and
start to be not suitable to be kept in the init/ directory anymore,
because they're pure mm concepts.

But it's not ideal either to keep the userfaultfd configs separate from
each other.  Hence this patch moves the userfaultfd configs under init/ to
be under mm/ so that we'll start to group all userfaultfd configs
together.

We do have quite a few examples of syscall related configs that are not
put under init/Kconfig: FTRACE_SYSCALLS, SWAP, FILE_LOCKING,
MEMFD_CREATE..  They all reside in the dir where they're more suitable for
the concept.  So it seems there's no restriction to keep the role of
having syscall related CONFIG_* under init/ only.

Link: https://lkml.kernel.org/r/20220420144823.35277-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

init/Kconfig

@@ -1667,16 +1667,6 @@ config ADVISE_SYSCALLS
 	  applications use these syscalls, you can disable this option to save
 	  space.
 
-config HAVE_ARCH_USERFAULTFD_WP
-	bool
-	help
-	  Arch has userfaultfd write protection support
-
-config HAVE_ARCH_USERFAULTFD_MINOR
-	bool
-	help
-	  Arch has userfaultfd minor fault support
-
 config MEMBARRIER
 	bool "Enable membarrier() system call" if EXPERT
 	default y
@@ -1741,13 +1731,6 @@ config KALLSYMS_BASE_RELATIVE
 
 # syscall, maps, verifier
 
-config USERFAULTFD
-	bool "Enable userfaultfd() system call"
-	depends on MMU
-	help
-	  Enable the userfaultfd() system call that allows to intercept and
-	  handle page faults in userland.
-
 config ARCH_HAS_MEMBARRIER_CALLBACKS
 	bool
 

mm/Kconfig

@@ -909,6 +909,23 @@ config ANON_VMA_NAME
 	  area from being merged with adjacent virtual memory areas due to the
 	  difference in their name.
 
+config USERFAULTFD
+	bool "Enable userfaultfd() system call"
+	depends on MMU
+	help
+	  Enable the userfaultfd() system call that allows to intercept and
+	  handle page faults in userland.
+
+config HAVE_ARCH_USERFAULTFD_WP
+	bool
+	help
+	  Arch has userfaultfd write protection support
+
+config HAVE_ARCH_USERFAULTFD_MINOR
+	bool
+	help
+	  Arch has userfaultfd minor fault support
+
 config PTE_MARKER
 	bool